The present invention relates to the manufacture of semiconductor devices. In particular, the invention relates to the manufacture of light-emitting diodes (LEDs) and related devices such as laser diodes formed in silicon carbide and related wide bandgap materials.
Silicon carbide is a preferred material for certain semiconductor devices, circuits and device precursors. Silicon carbide has a number of favorable physical and electronic properties that make its use attractive for devices in which relatively large bandgaps are desired or necessary. Because of its relatively wide bandgap and its greater recent availability in device quality crystals, wafers, substrates and epitaxial layers, silicon carbide has formed the foundation for a significant increase in the production, sale and use of LEDs that emit in the blue portion of the visible spectrum. Additionally, as other wide bandgap materials have more carefully adapted for light-emitting diode use, particularly the Group III nitrides, silicon carbide has proved to be an advantageous substrate material for Group III nitride-based light-emitting diodes.
One of the benefits of silicon carbide, in addition to its appropriate crystal structure match with many Group III nitrides, is the capability of silicon carbide to be conductively doped. Because silicon carbide can be conductively doped, a silicon carbide substrate can function as part of the current-carrying portion of a light-emitting diode. As a result, silicon carbide can form part of a “vertical” light-emitting diode; i.e., one in which the ohmic contacts are positioned on the top and bottom (i.e., opposite ends) of the device and thus direct the light-generating current to flow linearly through the device. As known to those familiar with other substrate materials (such as sapphire) which are not conductive, a vertical geometry device cannot be formed with an insulating or semi-insulating substrate. Instead, the respective ohmic contacts must be placed in a lateral relationship rather than a vertical one on the device. In most circumstances, the vertical orientation offers a number of advantages, including a proportionally smaller size, generally easier incorporation into circuits and packages, and resulting lower cost.
Given the desired function of a light-emitting diode, the structure of the device should enhance rather than hinder the light-emitting function. Furthermore, LEDs are often rated on the basis of light output (e.g. brightness in microWatts, μM) at a given current (e.g. milliamps, mA). Accordingly, when ohmic contacts are made to silicon carbide substrates, they are preferably added in a manner that minimizes the amount of the substrate that they cover in order to permit as much light as possible generated by the diode to be emitted through the substrate as well as in other directions.
Furthermore, in order to produce the ohmic contacts to silicon carbide, the preferred techniques and structures incorporate several layers of metal. For example, in the light emitting diodes and lasers just described, the backside (i.e., substrate) ohmic contact is often formed by depositing a first layer of nickel (Ni) and then overlaying the nickel layer with one or more additional layers that are (for example) selected combinations or alloys of titanium (Ti) and gold (Au), or titanium, platinum (Pt) and gold.
In LEDs available from the assignee of the present invention, the Ni and Ti/Au metallization layers are formed in a pattern resembling the letter “X” in order to minimize the surface area being covered. In other devices (e.g. power devices) large ohmic contacts can be advantageous, but in an LED it is desirable to avoid coating an entire side (front or back) with metal, because the ohmic metals absorb light and reduce the total light output, and thus the efficiency of the device.
As is quite familiar to those of ordinary skill in the semiconductor arts, commercial devices are often formed in large numbers on circular wafers of semiconductor materials. The term “wafer” is used herein in its usual sense to refer to an item that has a thickness that is small in comparison to essentially parallel large surface areas. The term “wafers” can include single crystal substrates, substrates with epilayers, or substrates carrying a large number of (usually identical) devices or circuits. In the discussions herein, the term wafer will usually refer to one carrying a large number of identical optoelectronic devices (usually LED's) formed from doped epilayers on a substrate, with respective ohmic contacts to each device.
After fabricating devices on a semiconductor wafer, the wafer is cut (“diced”) into individual chips, each of which contains a single device. Before packaging, each wafer should be inspected to ensure that the proper metallization layers have been deposited on the back side of the chip. If either the Ni or Ti/Au layer is missing, a good ohmic connection to the semiconductor substrate cannot be made. Alternately, even if an ohmic contact is made, poorly-formed layers can raise long-term reliability issues.
Presently, such inspection of SiC-based LEDs is performed manually and requires significant time and specialized equipment. The inspection also adds an additional, separate, unintegrated step to the manufacturing process. As a result, the inspection process may be somewhat inefficient and inaccurate. Furthermore, because the inspection process is manual, it cannot be easily combined with other manufacturing steps in a manner that would increase the overall efficiency of the process.
Nevertheless, identifying defective devices at an early stage avoids more expensive failure later on. Stated differently, identifying and discarding defective LEDs at the wafer stage is much less expensive than going to the additional cost of producing a fully packaged device which incorporates the defects that should have been identified earlier. Accordingly, early identification of the absence of the appropriate metal layers on the backside of a silicon carbide substrate is important.
Furthermore, silicon carbide wafers are relatively expensive. Accordingly, even when they are broken, if they contain possible useful die, these need to be inspected and the individual die or devices incorporated in the manufacturing stream if at all possible. As known to those familiar with semiconductor manufacturing, increasing the percentage of high quality devices on a wafer or in a process is one of the most fundamental ways to increase profitability.